TAN-046
Application Note - XRT73L04 4-Channel DS3/E3/STS-1
LIU IC Power Conditioning Requirements
May 23, 2001
Revision 1.03
2
analog ground. Do not insert any impedance (an inductor or ferrite bead) between analog
and digital ground pins of XRT73L04.
3.0 XRT 73L04 ‘POWER’ PINS
The XRT73L04 device has 17 power supply pins. Pins 78, 86, 95, and 103 are the
receive analog section power supply pins. Pins 4, 10, 13, 24, 27, 33, 47 and 134 are the
transmit analog section power supply pins. Finally, pins 51, 58, 122 and 128 are digital
power supplies. The receive analog power supply pins (pins 78, 86, 95 and 103) are the
most critical as they power the four (4) clock recovery phase-lock-loops and therefore
power to these pins should be as clean as possible.
It is also desirable to keep the transmit power supply noise isolated from the receive
power supply. Further, it is also desirable to keep the individual channels (within a given
XRT73L04 device) isolated from each other. Finally, if multiple XRT73L04 devices are
used in a system it is further desirable to keep their power isolated (from each other) in
order to minimize cross-talk.
The attached schematic (Figure 1) illustrates Exar’s recommendations on how to connect
the Analog and Digital VDD pins (of a single XRT73L04 device) to a 3.3V Power
Supply. Exar’s approach recommends the use of two-stage LC filtering.
3.1 STAGE 1 – A LARGE LC FILTER (consisting of L1 and C1)
This particular LC filter consists of a 15uH inductor and a 33uF capacitor. The purpose
of this LC filter is to eliminate much of the “DC-to-DC Converter-induced” low
frequency ripple, within the 3.3V power supply line, prior to being routed to any of the
VDD pins of the XRT73L04 device.
NOTES ABOUT THIS LC FILTER:
1. This LC filter should be placed close to the output of the DC-to-DC Converter.
2. Only one such LC filter is needed, per board (even in multi-channel applications).
However, multi-channel designs may have to use multiple instantiations of this
LC filter due to large voltage drops across the inductor (due to the increased
amount of current draw of multiple XRT73L04 devices and the dc resistance of
the inductor), or because of the maximum operating current limit of this inductor
as well.
3. This LC filter may not be necessary if the 3.3V Power Supply is already filtered
elsewhere in the system.